Sensor interrogation

ABSTRACT

A method of testing a system, which has at least one electrochemical sensor for detecting an analyte gas within a housing of the system, and the housing has an inlet, includes exhaling in the vicinity of the inlet of the housing of the system and measuring a response to exhaled breath to test one or more transport paths of the system. Measuring the response to exhaled breath may, for example, include measuring the response of a sensor within the housing of the system that is responsive to the presence of exhaled breath. The sensor responsive to the presence of exhaled breath may, for example, include an electrochemically active electrode responsive to a gas within exhaled breath. The electrochemically active electrode may, for example, be responsive to carbon dioxide or to oxygen.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 61/547,245, filed Oct. 14, 2011, and U.S. Provisional PatentApplication Ser. No. 61/698,153, filed Sep. 7, 2012, the disclosures ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The following information is provided to assist the reader inunderstanding certain technology including, for example, the devices,systems and/or methods disclosed below and representative environmentsin which such technology may be used. The terms used herein are notintended to be limited to any particular narrow interpretation unlessclearly stated otherwise in this document. References set forth hereinmay facilitate understanding of the technology or the backgroundthereof. The disclosure of all references cited herein are incorporatedby reference.

Prudence dictates that gas detection instrumentation be tested regularlyfor functionality. It is a common practice to, for example, perform a“bump check,” or functionality check on portable gas detectioninstrumentation on a daily basis. The purpose of this test is to ensurethe functionality of the entire gas detection system, commonly referredto as an instrument. A periodic bump check or functionality check mayalso be performed on a permanent gas detection instrument to, forexample, extend the period between full calibrations. Gas detectionsystems include at least one gas sensor, electronic circuitry and apower supply to drive the sensor, interpret its response and display itsresponse to the user. The systems further include a housing to encloseand protect such components. A bump check typically includes: a)applying a gas of interest (usually the target gas or analyte gas theinstrument is intended to detect); b) collecting and interpreting thesensor response; and c) indicating to the end user the functional stateof the system (that is, whether or not the instrument is properlyfunctioning).

Such bump tests are performed regularly and, typically, daily. Bumpchecks provide a relatively high degree of assurance to the user thatthe gas detection device is working properly. The bump check exercisesall the necessary functionalities of all parts of the gas detectiondevice in the same manner necessary to detect an alarm level of ahazardous gas. In that regard, the bump check ensures that there isefficient gas delivery from the outside of the instrument, through anytransport paths (including, for example, any protection and/or diffusionmembranes) to contact the active sensor components. The bump check alsoensures that the detection aspect of the sensor itself is workingproperly and that the sensor provides the proper response function orsignal. The bump check further ensures that the sensor is properlyconnected to its associated power supply and electronic circuitry andthat the sensor signal is interpreted properly. Moreover, the bump checkensures that the indicator(s) or user interface(s) (for example, adisplay and/or an annunciation functionality) of the gas detectioninstrument is/are functioning as intended.

However, a periodic/daily bump check requirement has a number ofsignificant drawbacks. For example, such bump checks are time consuming,especially in facilities that include many gas detection systems orinstruments. The bump check also requires the use of expensive andpotentially hazardous calibration gases. Further, the bump check alsorequires a specialized gas delivery system, usually including apressurized gas bottle, a pressure reducing regulator, and tubing andadapters to correctly supply the calibration gas to the instrument. Therequirement of a specialized gas delivery system often means that theopportunity to bump check a personal gas detection device is limited inplace and time by the availability of the gas delivery equipment.

SUMMARY OF THE INVENTION

In one aspect, a method of testing a system, which has at least oneelectrochemical sensor for detecting an analyte gas within a housing ofthe system, and the housing has an inlet, includes exhaling in thevicinity of the inlet of the housing of the system and measuring aresponse to exhaled breath to test one or more transport paths of thesystem. Measuring the response to exhaled breath may, for example,include measuring the response of a sensor within the housing of thesystem that is responsive to the presence of exhaled breath. The sensorresponsive to the presence of exhaled breath may, for example, includean electrochemically active electrode responsive to a gas within exhaledbreath. The electrochemically active electrode may, for example, beresponsive to carbon dioxide or to oxygen.

The method may further include simulating the presence of the analytegas electronically and measuring a response of the electrochemicalsensor to the electronic simulation. In a number of embodiments, aconstant current is caused to flow between a first working electrode anda counter electrode of the electrochemical sensor, and the measuredresponse is a potential difference. In a number of embodiments, aconstant potential difference is maintained between a first workingelectrode and a counter electrode of the electrochemical sensor, and themeasured response is a current. The electrochemical sensor may, forexample, be an amperometric sensor.

In a number of embodiments, the electrochemical sensor includes a firstworking electrode responsive to the analyte gas and a second workingelectrode responsive to a gas within exhaled breath. The electrochemicalsensor may, for example, include a sensor housing including at least oneinlet into an interior of the sensor housing wherein the first workingelectrode and the second working electrode are positioned within thesensor housing. Each of the first working electrode and the secondworking electrode may, for example, independently comprise anelectrocatalytically active material deposited upon a porous membranethrough which gas can diffuse.

In another aspect, a system includes a system housing, at least oneinlet formed in the system housing, at least one electrochemical sensorfor detecting an analyte gas within the system housing, and at least onesensor responsive to the presence of exhaled breath within the systemhousing. The sensor responsive to the presence of exhaled breath may,for example, include an electrochemically active electrode responsive toa gas within exhaled breath. The electrochemically active electrode may,for example, be responsive to carbon dioxide or to oxygen.

The system may further include a system to electronically interrogatethe electrochemical sensor. The system to electronically interrogate theelectrochemical sensor may, for example, include circuitry to simulatethe presence of the analyte gas electronically and to measure a responseof the electrochemical sensor to the electronic simulation. In a numberof embodiments, the circuitry is adapted to cause a constant current toflow between a first working electrode and a counter electrode of theelectrochemical sensor, and the measured response is a potentialdifference. In a number of embodiments, the circuitry is adapted tomaintain a constant potential difference between a first workingelectrode and a counter electrode of the electrochemical sensor and themeasured response is a current.

The electrochemical sensor may, for example, include a first workingelectrode responsive to the analyte gas and a second working electroderesponsive to a gas within exhaled breath. The electrochemical sensormay, for example, include a sensor housing including at least one inletinto an interior of the sensor housing, wherein the first workingelectrode and the second working electrode is positioned within thesensor housing. Each of the first working electrode and the secondworking electrode may, for example, independently include anelectrocatalytically active material deposited upon a porous membranethrough which gas can diffuse.

In a further aspect, a system for detecting at least one analyte gas,includes a system housing comprising an inlet system and anelectrochemical gas sensor within the housing and in fluid connectionwith the inlet system. The electrochemical sensor is responsive to theat least one analyte gas. The system further includes at least onesensor within the housing and in fluid connection with the inlet systemwhich is responsive to at least one driving force created in thevicinity of the inlet system other than by application of the at leastone analyte gas or a simulant gas to which the electrochemical sensor isresponsive to provide an indication of a state of a transport pathbetween the inlet system and the electrochemical gas sensor.

The driving force may, for example, be a change in the concentration ofa gas cause by exhalation of breath, a change in humidity, a change intemperature, a change in pressure, or a change in flow. In a number ofembodiments, the driving force is created by exhalation of breath in thevicinity of the inlet system.

In still a further aspect, a method of testing at least one transportpath in a system having a housing and an inlet in the housing, wherein aprimary function of the system is other than to measure a property ofexhaled breath; includes exhaling in the vicinity of the inlet of thehousing and measuring a response to exhaled breath to test the at leastone transport path of the system.

The present invention, along with the attributes and attendantadvantages thereof, will best be appreciated and understood in view ofthe following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a user exhaling in a manner that the user's exhaledbreath impinges upon an inlet of a system including a housing enclosinga sensor that is sensitive to at least one property of exhaled breath.

FIG. 2A illustrates a schematic, cross-sectional view of an embodimentof a system or instrument including at least one sensor which includes afirst working electrode sensitive or responsive to an analyte and asecond electrode sensitive or responsive to a driving force associated,for example, with the presence of exhaled breath.

FIG. 2B illustrates an enlarged side, cross-sectional view of a portionof the sensor of FIG. 2A including a housing lid in which a gas inlethole is formed to be in fluid connection with a gas diffusion space anda porous gas diffusion membrane, wherein the first working electrode andthe second working electrode are formed on or attached to an interiorside of the diffusion membrane.

FIG. 2C illustrates a bottom view of the portion of the sensorillustrated in FIG. 2B.

FIG. 3A illustrates a perspective exploded view of another embodiment ofa sensor including a first working electrode sensitive or responsive toan analyte and a second electrode sensitive or responsive to thepresence of exhaled breath, wherein the first working electrode isformed on a first diffusion membrane and the second working electrode isformed on a second diffusion membrane.

FIG. 3B illustrates a cross-sectional view of the sensor of FIG. 3Awithin an instrument or system housing.

FIG. 3C illustrates an enlarged side, cross-sectional view of a portionof the sensor of FIG. 3A including a housing lid in which two gas inletholes are formed, wherein each of the first working electrode and thesecond working electrode are in general alignment with one of the twogas inlet holes.

FIG. 3D illustrates a bottom view of the portion of the sensorillustrated in FIG. 3C.

FIG. 3E illustrates a schematic, cross-sectional view of anotherembodiment of a sensor including a first working electrode sensitive orresponsive to an analyte and a second electrode sensitive or responsiveto the presence of exhaled breath, wherein the first working electrodeis formed on a first diffusion membrane positioned in a first cell andthe second working electrode is formed on a second diffusion membranepositioned in a second cell.

FIG. 3F illustrates a side, cross-sectional view of a portion of anotherembodiment of a sensor including a housing having an inlet in the formof an extending slot and a diffusion member in fluid connection with theinlet.

FIG. 3G illustrates a top view of the sensor of FIG. 3F.

FIG. 4 illustrates a study of the response of the sensor of FIG. 3A,wherein the first working electrode is sensitive to hydrogen sulfide andthe second working electrode is sensitive to oxygen, when challengedwith exhaled breath, followed by a mixture of 15 vol-% oxygen and 20 ppmhydrogen sulfide, followed by nitrogen.

FIG. 5A illustrates a ribbon and a wire which may be used to form sensorelements in the systems hereof, which is adapted to measure a responseto, for example, exhaled breath to test one or more transport paths ofthe system.

FIG. 5B illustrates sensor elements hereof including a conductive ribbonand a conductive wire upon which an electrocatalytic material is coatedor immobilized.

FIG. 5C illustrates a sensor element hereof including an extendingribbon having a rectangular end member which is wider than the extendingribbon

FIG. 5D illustrates the sensor element of FIG. 5C having anelectrocatalytic material immobilized on the end member thereof.

FIG. 5E illustrates a sensor element hereof including an extending wirehaving a spiraled section on an end thereof

FIG. 5F the sensor element of FIG. 5E including an electrocatalyticmaterial immobilized on the spiraled section thereof.

FIG. 6 illustrates an embodiment of an interdigitated electrode systemhereof wherein a first branch of the electrode system includes a firstelectrocatalytic material and a second branch includes a secondelectrocatalytic material.

FIG. 7 illustrates an embodiment of an electrode system hereof wherein afirst electrode and a second electrode are supported upon a gas porousdisk, which is formed as an annulus.

FIG. 8 illustrates the response of a representative example of a singlechannel amperometric sensor hereof having a single electrode fabricatedto include an electrocatalytic material that is responsive to ananalyte, to exhaled breath and to nitrogen

FIG. 9 illustrates a decision tree diagram setting forth arepresentative embodiment of an operating mode or method of a systemhereof.

FIG. 10 illustrates an equivalent circuit used to describeelectrochemical cells.

FIG. 11 illustrates a block diagram of an embodiment of measurementcircuitry for electronic interrogation.

DETAILED DESCRIPTION OF THE INVENTION

As used herein and in the appended claims, the singular forms “a,” “an”,and “the” include plural references unless the context clearly dictatesotherwise. Thus, for example, reference to “a transport path” includes aplurality of such transport paths and equivalents thereof known to thoseskilled in the art, and so forth, and reference to “the transport path”is a reference to one or more such transport paths and equivalentsthereof known to those skilled in the art, and so forth.

As, for example, illustrated schematically in FIG. 1, in a number ofembodiments, the devices, systems and/or methods hereof are operable totest transport properties of a gas detection or other system 10 viaapplication of a driving force other than an analyte gas or a simulantgas (that is, a gas simulating the analyte gas by evoking a responsefrom the analytical electrode of the system) from a container to one ormore inlets 22 of an enclosing housing 20 of system 10. In a number ofembodiments, the driving force may, for example, be the application ofexhaled breath to inlet(s) 22. Housing 20 may, for example, include amass transport path into an interior thereof (for example, a diffusionpath) in fluid connection with inlet 22. The path may, for example,include or be in fluid connection with a mass transport or diffusionmember or barrier 30 (for example, a membrane through which gas ismobile (for example, via diffusion) but through which a liquid haslimited or no mobility). Housing 20 encloses a sensor 40 which issensitive to the presence of exhaled breath. For example, sensor 40 maybe sensitive to an environmental gas (the concentration of which ischanged by the presence of exhaled breath), to a gas within exhaledbreath, to a change in humidity, to a change in temperature, to a changein pressure, to a change in flow etc. A response of sensor 40 to exhaledbreath provides a measurement of the transport properties and/orfunctionality of one or more transport paths of system 10.

In a number of representative embodiments discussed herein, devices,systems and/or methods hereof decrease or eliminate the necessity tobump check a gas detection instrument with stored calibration (forexample, an analyte or a simulant) gas. Such representative embodimentsof systems, devices and/or methods may, for example, combine aninternal, electronic check or interrogation of sensor functionality,connection, and/or correction (as, for example, described in U.S. Pat.No. 7,413,645) with a transport path test using a “secondary” sensorsensitive to a driving force other than the presence of an analyte gasor a simulant gas (for example, a driving force/variable change arisingfrom the presence of exhaled human breath as described above).

Many gas detection devices, instruments or systems (for example,portable gas detection instruments) include amperometric electrochemicalgas sensors. These sensors are often referred to as “fuel cell” typesensors, which refers to a primary principle of operation. Suchelectrochemical gas sensors are typically combined or integrated into adevice, system or instrument with a battery or other power supply,appropriate electronic driving circuitry (for example, including apotentiostat), a display, and one or more alarms (or other means ofcommunicating to the user the presence of a dangerous level of harmfulor toxic gas or a condition of dangerous oxygen depletion orenrichment). The sensor, circuitry and displays are typically containedin a rugged, sealed housing. As used in connection with such aninstrument, the term “sealed” refers to protection of the sensor,circuitry, and displays from harmful environmental hazards (for example,dusts, condensing vapors, such as paints or coatings, and water and/orother liquids). However, the sealed housing must continually provide forthe efficient transfer of the target or analyte gas(es) from outside theinstrument housing into a housing of the sensor itself. Often, thisresult is accomplished with one or more porous diffusion membranes thatkeep dusts, vapors, and liquids out of the instrument housing, but allowone or more analyte gases of interest to be transported into the sensoritself. This transport is typically accomplished by gaseous diffusion orby pumping an analyte gas stream into or across the face of the sensor.

As described above, the need to bump check a gas detection system/devicewith a calibration or simulant gas from a container is decreased oreliminated by providing a sensor (for example, a secondary sensor) thatis sensitive to or responds to a driving force or variable change in thevicinity of the inlet of the system, such as, for example, the presenceof exhaled breath. In a number of embodiments, components which make asensor responsive to oxygen are provided in an amperometricelectrochemical sensor (which is functional to detect an analyte otherthan oxygen). Exhaled human breath typically includes 4 to 5volume-percent (vol-%) of carbon dioxide (CO₂) and 15.8 to 16.8 vol-%oxygen (O₂). In contrast, ambient air includes approximately 20.8 vol-%O₂ and 0.035 vol-% CO₂. Thus, when a user exhales in the vicinity of oneor more inlets into the housing of the detection system or instrument,the exhaled breath displaces the volume of gas (ambient air) within adiffusion volume in a sensor therein with the exhaled breath. A responseto the decreased concentration of oxygen in exhaled breath as comparedto ambient air may be used to test the transport properties of whatevergas transport path or mechanism may be used in the gas detection device(for example, including one or more gas diffusion membranes). The sameresult may, for example, be accomplished by incorporating, within oralong with, for example, a toxic gas, a combustible or other sensorchannel, a sensing element (which may be the same as or different fromthe sensing element for the analyte) that responds to any or allcomponents of exhaled breath. For example, a similar result may beobtained by including a sensor or sensing functionality that responds tothe increased concentration of CO₂ in exhaled breath as compared toambient air. In that regard, exhaled breath contains approximately 5 vol% CO₂, as compared to ambient air, which contains approximately 600 ppmCO₂ (0.06 vol-%). A sensor or sensing system to measure CO₂concentration may, for example, include an electrochemical sensor and/ora non-dispersive infrared sensor.

Amperometric or fuel cell-type gas sensors typically include at leasttwo electrocatalytic electrodes (an anode and a cathode), at least oneof which is a gas diffusion electrode or working electrode. The workingelectrode can be either the anode or the cathode in any given sensor.The gas diffusion electrode typically includes fine particles of anelectrocatalytic material adhered to one side of a porous orgas-permeable membrane.

The electrocatalytic side of the working electrode is in ionic contactwith the second electrode (the counter electrode, whether the anode orthe cathode) via an electrolyte (for example, a liquid electrolyte, asolid electrolyte, a quasi-solid state electrolyte or an ionic liquid).A liquid electrolyte is typically a solution of a strong electrolytesalt dissolved in a suitable solvent, such as water. An organic solventmay also be used. Quasi-solid state electrolytes can, for example,include a liquid electrolyte immobilized by a high-surface-area,high-pore-volume solid. The working electrode and the counter electrodeare also in electrical contact via an external circuit used to measurethe current that flows through the sensor.

Additionally, although by no means necessary, a third or referenceelectrode, is often included. The reference electrode is constructed ina way that its potential is relatively invariant over commonly occurringenvironmental conditions. The reference electrode serves as a fixedpoint in potential space against which the operating potential of theworking electrode may be fixed. In this way, electrochemical reactionsthat would not normally be accessible may be used to detect the analytegas of interest. This result may be accomplished via control and drivingcircuitry which may, for example, include a potentiostat.

FIGS. 2A through 2C illustrate a schematic diagram of an instrument orsystem 100 including at least one electrochemical sensor or sensorsystem 110. System 100 includes a system housing 102 including an inletor inlet system 104 which places an interior of system housing 102 influid connection with the ambient environment. Electrochemical sensorsystem 110 includes at least one primary sensor responsive to at leastone analyte gas. System 100 further includes at least one secondarysensor which is responsive to a driving force or variable change outsideof system housing 102 in the vicinity of inlet 104 other than a changein concentration of the analyte gas or a simulant gas (that is, a gasother than the analyte gas to which the primary sensor is responsive)applied to system 100 from a container. A system 50 for creating such adriving force or variable change is illustrated schematically in FIG.2A. System 50 may, for example, change the concentration of a gas,change humidity, change temperature, change pressure, change flow etc.in the vicinity of system inlet 104. The secondary sensor is responsiveto the driving force created by system 50. The response of the secondarysensor to the driving force is indicative of the state of the path ortransport path between inlet 104 and the secondary sensor. In general,the transport path is the path via which a gas is transported fromoutside housing 102 (via inlet 104) to the secondary sensor (whether by,for example, diffusion or pumping). The transport path between inlet 104and the secondary sensor and the transport path between inlet 104 andthe primary sensor may, for example, be the same or similar and areexposed to generally the same conditions over the life of system 100.The secondary sensor may, for example, be positioned in close proximityto the primary sensor. The response of the secondary sensor to thedriving forces provides an indication of the state of the transportbetween system inlet 104 and the primary sensor.

In a number of representative embodiments described herein, system 50represents a person who exhales in the vicinity of inlet 104. In thecase of exhaled breath, the driving force may be any one of (or morethan one of) a change in the concentration of a gas (for example, oxygenor carbon dioxide), a change in humidity, a change in temperature, achange in pressure, or a change in flow. The secondary sensor may thusinclude a gas sensor, a humidity sensor, a temperature sensor, apressure sensor and/or a flow sensor. In the case that, for example, thesecondary sensor is a humidity sensor, a temperature sensor, a pressuresensor or a flow sensor, system 50 need not be a person who exhales inthe vicinity of system inlet 104. System 50 may, for example, be anysystem or device suitable to create a change in humidity, a change intemperature, a change in pressure, or a change in flow. The degree ofchange in the variable of interest may, for example, be controlled tomonitor for a corresponding response of the secondary sensor. In thecase of a change in temperature, system 50 may, for example, including aheating element. In the case of a change in pressure or a change inflow, system 50 may, for example, include a small, manually operated airpump such as a bellows.

In a number of representative embodiments hereof, the secondary sensorincludes a gas sensor responsive to the concentration of a gas which ischanged by exhalation in the vicinity of system inlet 104. In severalsuch embodiments, sensor 110 includes a housing 120 having a gas inlet130 (formed in a lid 122 of sensor housing 120) for entry of analyte gasand human breath into sensor 110. In the illustrated embodiment, inlet130 is in fluid connection with a gas diffusion volume or space 118.Electrolyte saturated wick materials 140 a, 140 b and 140 c separate afirst working electrode 150 a (responsive to the presence of analytegas) and a second working electrode 150 b (responsive to the presence ofhuman breath) from reference electrode(s) 170 and counter electrode(s)180 within sensor 110 and provide ionic conduction therebetween via theelectrolyte absorbed therein. First working electrode 150 a, referenceelectrode 170 and counter electrode 180, in cooperation with electrolytesaturated wick materials 140 a, 140 b and 140 c form a portion of theprimary sensor. Second working electrode 150 b, reference electrode 170and counter electrode 180, in cooperation with electrolyte saturatedwick materials 140 a, 140 b and 140 c form a portion of the secondarysensor. Electronic circuitry 190 as known in the art is provided, forexample, to maintain a desired potential between working electrodes 150a and 150 b and reference electrode(s) 170, to process an output signalfrom sensor 110 and to connect/communicate with other components ofsystem 100 (including, for example, one or more displays, communicationsystems, power supplies etc.).

In the illustrated embodiment, first working electrode 150 a and secondworking electrode 150 b are located to be generally coplanar withinsensor housing 120. In the illustrated embodiment, first workingelectrode 150 a is formed by depositing a first layer of catalyst 154 aon a diffusion membrane 152 (using, for example, catalyst depositiontechnique known in the sensor arts). Second working electrode 150 b isalso formed by depositing a second layer of catalyst 154 b on diffusionmembrane 152 (using, for example, catalyst deposition techniques knownin the sensor arts). Methods of fabricating electrodes on diffusionmembranes are, for example, described in U.S. Patent ApplicationPublication No. 2011/0100813. Catalyst layers 152 a and 152 b may or maynot be formed using the same electrocatalytic material. It is immaterialwhether second gas diffusion or working electrode 150 b is operated asan anode or cathode with respect to the operation of first gas diffusionor working electrode 150 a.

FIGS. 3A through 3D illustrate an embodiment of a sensor 210 that issimilar in design and operation to sensor 110. Like elements of sensor210 are numbered similarly to corresponding elements of sensor 110 withthe addition of 100 to the reference numbers of the elements of sensor210. As illustrated in FIG. 3A, reference electrode 270, counterelectrode 280 and electrolyte absorbent wicks 240 a, 240 b and 240 c aresupported within housing 220 via a support member 284. A printed circuitboard 292 is connected to housing 220 and may form a part of theelectronic circuitry of sensor 210.

As, for example, illustrated in FIGS. 3A and 3C, a housing lid 222includes a first gas inlet 230 a and a second gas inlet 230 b. First gasinlet 230 a and a second gas inlet 230 b may, for example, be in fluidconnection with an inlet system 204 (including, for example, one or moreinlets) formed in a housing 202 of an instrument or system 200 (see FIG.3B). First inlet 230 a can, for example, be designed for use inconnection with a first working electrode 250 a for an analyte gas suchas hydrogen sulfide. A first catalyst layer 254 a of first workingelectrode 254 a, which is deposited upon a first diffusion membrane 252a, may, for example, include iridium in the case that the analyte gas ishydrogen sulfide (H₂S). Second inlet 230 b is designed for use inconnection with the application of exhaled breath to second workingelectrode 250 b. Second working electrode 250 b is formed by depositionof a second catalyst layer 254 b upon a second diffusion membrane 252 b.Separate gas inlets 230 a and 230 b may, for example, be designed oroptimized for passage of two different gases. In that regard, first gasinlet 230 a may be optimized (for example, in dimension and/or shape)for the analyte gas of interest, while second gas inlet 230 b may beoptimized for a component of exhaled breath.

In the case of an aqueous electrolyte, the material(s) (which can be thesame or different) of the gas diffusion membranes can be generallyhydrophobic in nature to minimize or eliminate any flow of the aqueouselectrolyte therethrough. In the case of a non-aqueous (for example,organic) electrolyte, the material of the gas diffusion membranes can begenerally oleophobic in nature to minimize or eliminate any flow of thenon-aqueous electrolyte therethrough. The material(s) can also behydrophobic and oleophobic. Such materials are referred to as“multiphobic”. The materials can also be chemically or otherwise treatedto minimize or eliminate liquid electrolyte flow or leakagetherethrough.

In general, the term “hydrophobic” as used herein refers to materialsthat are substantially or completely resistant to wetting by water atpressures experienced within electrochemical sensors (and thus limitflow of aqueous electrolyte therethrough). In general, the term“oleophobic” as used herein refers to materials that are substantiallyor completely resistant to wetting by low-surface tension liquids suchas non-aqueous electrolyte systems at pressures experienced withinelectrochemical sensors (and thus limit flow of non-aqueous electrolytetherethrough). As used herein, the phrase “low-surface tension liquids”refers generally to liquids having a surface tension less than that ofwater. Hydrophobic, oleophobic, and multiphobic materials for use inelectrodes are, for example, discussed in U.S. Pat. No. 5,944,969.

Gas diffusion membranes for use herein can, for example, be formed frompolymeric materials such as, but not limited to, polytetrafluoroethylene(for example, GORETEX®), polyethylene or polyvinylidene fluoride (PVDF).Such polymeric materials can, for example, include a pore structuretherein that provides for gas diffusion therethrough.

In sensors 110 and 210, first working electrodes 150 a and 250 a share acommon electrolyte, a common counter electrode (180 and 280) and acommon reference electrode (170 and 270) with second working electrodes150 b and 250 b, respectively. In certain situations, depending, forexample, upon the analyte gas to be detected and the associatedelectrochemistry, it may not be desirable or possible to have a commonelectrolyte, counter electrode and/or reference electrode. FIG. 3Eillustrates another embodiment of a sensor 210′, which is similar inoperation and construction to sensors 110 and 210. Unlike sensors 110and 210, in the embodiment of 210′, first working electrode 250 a′ andsecond working electrode 250 b′ are positioned in separate cells withinhousing 120″ which are not in fluid connection. In this manner, adifferent electrolyte can be used in connection with electrolytesaturated wick materials 140 a′, 140 b′ and 140 c′ than the electrolyteused in connection with electrolyte saturated wick materials 140 a″, 140b″ and 140 c″. Likewise, reference electrode 170 a′ may be formeddifferently from reference electrode 170 b′, and/or counter electrode180 a′ may be formed differently from counter electrode 180 b′. In theillustrated embodiment, separate inlets 230 a′ and 230 b′ are formed ina common lid or cap 222′ to be in fluid connection with first workingelectrode 250 a′ and second working electrode 250 b′, respectively.

FIGS. 3F and 3G illustrates another embodiment of a sensor 310, which issimilar in operation and construction to sensors 110 and 210. Sensor 310includes a housing 320 having a gas inlet 330 (formed in a lid 322 ofsensor housing 320) for entry of analyte gas and human breath intosensor 110. In the illustrated embodiment, inlet 330 is formed as anextending slot in lid 322 and is in fluid connection with a gasdiffusion member 318. Gas diffusion member 318 is, for example, formedfrom a porous polymeric material and provides for relatively quicklateral diffusion of gas to a first working electrode 350 a (responsiveto the presence of analyte gas) and a second working electrode 350 b(responsive to the presence of human breath) to reduce response times ofsensor 310. First working electrode 350 a, second working electrode 350b, and remainder of the components of sensor 330, may, for example, beformed in the same manner as described above for working electrode 150a, second working electrode 150 b and the remainder of the components ofsensor 110. Gas diffusion member 318 may, for example, be stiffer inconstruction than diffusion membrane 352 a of first working electrode350 a and diffusion membrane 352 b of second working electrode 350 b(upon which, catalyst layers 354 a and 354 b, respectively, aredeposited). In addition to providing relatively quick lateral diffusion,gas diffusion member 318 may also protect diffusion membranes 352 a and352 b from “pinching” as a result of mechanical compression.

Although the transport paths for first working electrodes 250 a, 250 a′and 350 a and for second working electrodes 250 b, 250 b′ and 350 b ofsensor 210, 210′ and 310 are slightly different, all transport paths ina particular instrument experience generally the same environments andenvironmental conditions. Therefore, a challenge with exhaled breath andthe measured response of second working electrodes 250 b, 250 b′ and 350b thereto provides an indication of the functionality of all transportpaths in the system or instrument.

In several studies of sensors fabricated in the manner of sensor 210hereof, first gas diffusion or working electrode 250 a was used todetect hydrogen sulfide (H₂S), while second gas diffusion or workingelectrode 250 b was used to detect the oxygen component of exhaledbreath. Sensors fabricated in the manner of sensor 110, sensor 210′ orsensor 310 would operate in the same manner. In the specifically studiedembodiments, first electrocatalyst layer 254 a included iridium (Ir)metal. Second electrocatalyst layer 254 b included platinum (Pt) metal,Other electrocatalysts suitable for reduction of oxygen may be used insecond electrocatalyst layer 254 b.

FIG. 4 illustrates the behavior sensor 210 when challenged with exhaledbreath, followed by a mixture of 15 vol-% oxygen and 20 ppm hydrogensulfide, followed by nitrogen. The H₂S channel trace is the response offirst working electrode 250 a (designed to detect hydrogen sulfide), andthe O₂ channel trace is the response of second working electrode 250 b(designed to detect the oxygen component of exhaled breath). Asillustrated, second working electrode 250 b responds to the decreasedoxygen content of exhaled breath which occurs at approximately the 50second mark in the graph. A mixture of 15 vol-% oxygen and 20 ppmhydrogen sulfide was applied at approximately 100 seconds. Each of firstworking electrode 250 a and second working electrode 250 b respondedappropriately to this challenge gas. Finally, nitrogen was applied at250 seconds. Upon application of nitrogen, second working electrode 250b (designed for the detection of oxygen) responded appropriately to thechallenge gas.

The response of second working electrode 250 b to exhaled breath asshown in FIG. 4 may, for example, be used to determine that thetransport paths (including gas diffusion members and/or membranes) of aportable gas detection instrument are, for example, not compromised bydust, vapors, and/or liquid. That is, based on the response of secondworking electrode 250 b to the decreased oxygen concentration of exhaledbreath, it can be determined that there is appropriate flow through allgas diffusion members (for example, gas diffusion membranes 252 a and252 b), whether they are part of sensor 210 itself or part of theoverall instrument. This gas response, when combined with an internalelectronic interrogation signal such as that described in U.S. Pat. No.7,413,645, may be used to provide a check of both the internalconductive condition of an amperometric electrochemical sensor and anygas transport path(s) (including, for example, associated gas diffusionmembranes), whether part of the sensor cell itself or part of theoverall instrument. In this manner, a test similar in overall result toa bump test is accomplished without the use of expensive and potentiallyhazardous calibration gas and equipment associated therewith.

In a number of embodiments hereof for use in connection with an exhaledbreath test or bump check, an amperometric oxygen (or other gas) sensingelement is disposed within, for example, an amperometric toxic (orother) gas sensor for detecting an analyte of interest. In a number ofthe embodiments described above, both an analyte gas sensing workingelectrode and the oxygen sensing electrode are conventionally fabricatedas gas diffusion electrodes. In many cases, such gas diffusionelectrodes include a high surface area electrocatalyst dispersed on aporous support membrane. In embodiments in which an amperometric gassensor is used in systems hereof as a secondary sensor to test one ormore transport paths, because the secondary sensor (for example, anoxygen sensor) is not used to present an analytical signal, there may beno need to use either a gas diffusion electrode or a high surface areaelectrocatalyst.

For example, a conductor such as a contact ribbon or another conductivemember, which are often used to carry an electrical signal from a gasdiffusion electrode, may have sufficient surface area andelectrocatalytic activity to be used as an oxygen, CO₂ or other gassensitive electrode. For example, FIG. 5A illustrates a ribbon 450 a anda wire 450 a′ which may be used to form a non-analytical sensor elementin the systems hereof. Such ribbons or wires may, for example, befabricated from an electrocatalytic material such as Platinum (Pt),Iridium (Ir), Gold (Au) or carbon (C). As illustrated in FIG. 5B sensorelements 550 a and 550 a′ hereof may, for example, be a conductiveribbon 552 a or a conductive wire 552 a′, respectively, upon which anelectrocatalytic material 554 a and 554 a′ (for example, Pt, Ir, Au, Cetc.), respectively, is coated or immobilized. The material of ribbon552 a and wire 552 a′ may be the same or different from electrocatalyticmaterial 554 a and 554 a′ immobilized thereon.

The sensor elements or electrodes hereof for testing transport paths maytake a wide variety of two-dimensional or three-dimensional shapes. Forexample, FIG. 5C illustrates a sensor element 650 a hereof including anextending ribbon 652 a having a rectangular end member 653 a which iswider than extending ribbon 652 a to, for example, provide increasedsurface area per unit length as compared to a ribbon of the same length.Similarly, FIG. 5D illustrates a sensor element 650 a′ hereof includingan extending ribbon 652 a′ having a rectangular end member 653 a. In theembodiment of FIG. 5D, an electrocatalytic material 654 a′ isimmobilized on end member 653 a. FIG. 5E illustrates a sensor element750 a hereof including an extending wire 752 a having a spiraled section653 a on an end thereof, which may, for example, provide increasedsurface area per unit length as compared to an extending wire of thesame length. Similarly, FIG. 5F illustrates a sensor element 750 a′hereof including an extending wire 752 a′ having a spiraled section 653a on an end thereof. In the embodiment of FIG. 5F, an electrocatalyticmaterial 754 a′ is immobilized on spiraled section 753 a′. In theembodiments of FIGS. 5D and 5F, electrocatalytic materials 654 a′ and754 a′ may be the same or different as the material upon which theelectrocatalytic material is immobilized.

In the embodiments discussed above, a first electrode is used forsensing an analyte and a second electrode, formed separately from thefirst electrode, is used to, for example, detect oxygen concentration.In the representative example of a toxic gas sensor for detecting theanalyte H₂S, for example, the toxic gas channel (H₂S, in that case) isfabricated to include the electrocatalyst iridium (Ir) and theoxygen-sensing electrode is fabricated to include the electrocatalystplatinum (Pt). Those electrocatalysts may, for example, be independentlydispersed onto the same porous substrate, but in two distinct anddifferent areas. The same or similar functionality may, for example, beachieved if mixtures of Pt and Ir are used. For example, such mixturesmay be physical mixtures of high surface area catalytic powders or suchmixtures may be alloys. In a number of embodiments, one electrocatalyticsubstance or material may, for example, be fabricated on top of anotherelectrocatalytic substance or material in a two-step process.

Moreover, the two electrocatalytic materials may, for example, befabricated into an interdigitated electrode system. FIG. 6 illustratesan embodiment of an interdigitated electrode system 850 wherein a firstbranch 850 a of electrode system 850 includes a first electrocatalyticmaterial and a second branch 850 b includes a second electrocatalytic.The first and second electrocatalytic materials of the two branches or“fingers” 850 a and 850 b of electrode system 850 may, for example, befabricated to include the same electrocatalytic substance (or mixture ofsubstances) or to include different electrocatalytic substances.

In another embodiment of an electrode system 950 hereof illustrated inFIG. 7, a first electrode 950 a and a second electrode 950 b aresupported upon a gas porous disk 960, which is formed as an annulus inthe illustrated embodiment. Disk 960 may, for example, be fabricatedfrom gas porous or permeable (that is, adapted to transport gastherethrough) polymer or another material that is inert in theelectrolyte used in the sensor system. As described above, disk 960serves as an electrode support onto which first working electrode 950 aand secondary working electrodes 950 b are fabricated, but on oppositesides of disk 960 as illustrated in FIG. 7. First or upper electrode 950a (in the orientation of FIG. 7) is formed as an annulus. Second orbottom electrode 950 b is formed as a disk centered on the annulus ofdisk 960. Electrode system 950 further includes a first or upperelectrolyte wick 970 a and a second or lower electrolyte wick 970 b.Electrode system also includes a first electrode current collector 980 aand a second electrode current collector 980 b.

The configuration of FIG. 7 may, for example, be vertically flipped orrotated 180° from its illustrated orientation and still function asintended. Many other shapes and configuration of electrodes are possiblefor use herein. Moreover, electrodes hereof may, for example, be stackedin multiple layers or other arrangements to produce sensors with asensitivity for a multiplicity of target gases.

In a number of embodiments hereof, a single working or sensing electrodecan be used which responds to both the analytical gas of interest(analyte) and to a another driving force (for example, a component ofexhaled breath) to enable testing of one or more transport paths to theelectrode(s) of the system. For example, in the representative sensorsystem described in FIG. 2, the H₂S working electrode also responds toexhaled breath. The response of the working electrode to exhaled breathcan be used to test the function of the transport path. FIG. 8illustrates the response of a representative example of a single channelamperometric sensor having a single electrode fabricated to include anelectrocatalytic material that is responsive to an analytical gas ofinterest or analyte (H₂S in the representative example), to exhaledbreath and to nitrogen. The electrode may be fabricated from a singleelectrocatalytic material, a physical mixture of electrocatalyticmaterials or an alloy of electrocatalytic materials.

In a number of embodiments of sensor systems hereof, two sensing orworking electrodes are provided which include the same electrocatalyticmaterial immobilized thereon. The electrodes can, for example, befabricated in an identical manner. In such embodiments, the analyte gasand, for example, a gas of interest in exhaled breath are eachelectroactive on the electrocatalytic material. In a number ofembodiments, the function of the two electrodes is alternated (forexample, each time the user activates a breath check as describedabove). Referring to, for example, FIG. 6, the first and secondelectrocatalytic materials of the two branches or electrodes 850 a and850 b of electrode system 850 would include the same electrocatalyticmaterial. In a first instance of activation of the instrument includingelectrodes 850 a and 850 b, electrode 850 a would be used as the workingelectrode for the target analyte gas and electrode 850 b would be usedto, for example, detect a component of exhaled breath (for example,oxygen). The next time the user activates the internal breath check (asecond instance), the functions of electrodes 850 a and 850 b would beswitched by the external circuitry and logic of the system or instrumentincluding sensors 850 a and 850 b. That is, in the second instance,electrode 850 b would be used as the working electrode for the targetanalyte gas and electrode 850 a would be used to detect the component ofexhaled breath. In this manner, alternatively, each electrode area wouldbe calibrated against the target gas of interest and the electronic lifeand health checks described below would be periodically applied to eachelectrode. Such a system and methodology provides a greater amount ofsurveillance and surety to the test methodology. A detection or sensingelement switching scheme which may be adapted for user herein isdescribed in U.S. Patent Application Publication No. 2011/0100090, thedisclosure of which is incorporated herein by reference.

In the case that oxygen variation (for example, as a result of a breathtest) is measured, sensing elements other than amperometric oxygensensing element may, for example, be used. In that regard, anyalternative oxygen sensing system may be used in place of anamperometric oxygen sensing. Representative examples of suitable oxygensensing systems include, but are not limited to, a metal oxidesemiconductor or MOS (also colloquially referred to as a “Figaro”sensor) oxygen sensing element, a high temperature potentiometric oxygensensor (zirconia sensor), or a paramagnetic oxygen sensor. A particularoxygen sensing technology may, for example, be more suitable as acomplement to a given toxic gas or other sensing technology for aparticular use. For example, an MOS or zirconia-based oxygen sensingelement may be well suited for use with an MOS toxic sensor or with aheated catalytic bead combustible gas sensor.

FIG. 9 illustrates a decision tree diagram that depicts a representativeembodiment of an operating mode or method hereof. The method illustratedin FIG. 9 assumes a successful complete calibration of the instrument(with a calibration gas) at some point in time, either at final assemblyand testing or in the field. In daily use, when the instrument is turnedon, as is typical, the instrument will perform its necessaryself-diagnosis checks. Part of this self-diagnosis may, for example,include the application of a life and health check similar to thatdescribed in U.S. Pat. No. 7,413,645.

As described in U.S. Pat. No. 7,413,645, and as illustrated in FIG. 10,a sensor generally can be described as a combination of resistances andcapacitance in series. The resistance R_(R) resulting from the referenceelectrode of FIG. 10 is not part of the current path of the analyticalsignal of the sensor. The resistive portion of this circuit is primarilya result of the solution (ionic) resistance of the electrolyteinterspersed between the working electrode (R_(W)) and the counterelectrode (R_(C)). The capacitive portion (C_(W)) of the equivalentcircuit is primarily a result of the micro solution environment foundvery close to the surfaces of the metallic particles that comprise theworking electrode. As a result of electrostatic forces, the volume ofsolution very close to the electrode surface is a very highly orderedstructure. This structure is important to understanding electrodeprocesses. The volume of solution very close to the electrode surface isvariously referred to as the diffusion layer, diffuse layer, and or theHelmholtz layer or plane.

The magnitudes of the resistance and capacitance present in anelectrochemical cell are a result of the nature and identities of thematerials used in its fabrication. The resistance of the electrolyte isa result of the number and types of ions dissolved in the solvent. Thecapacitance of the electrode is primarily a function of the effectivesurface area of the electrocatalyst. In an ideal world, these quantitiesare invariant. However, the solution resistance present in anamperometric gas sensor that utilizes an aqueous (water-based)electrolyte may change, for example, as a result of exposure todifferent ambient relative humidity levels. As water transpires from thesensor, the chemical concentration of the ionic electrolyte increases.This concentration change can lead to increases or decreases in theresistivity of the electrolyte, depending on the actual electrolyteused.

The response curves of sensors have the shape expected for the chargingcurve of a capacitor, that is a typical “RC” curve. In a number ofembodiments, the analytical signal used to determine the “health” of asensor is the algebraic difference in the observed potential just priorto the application of a current pulse and at the end of the currentpulse. The magnitude of the potential difference observed as a functionof the application of the current pulse is an indicator of the presenceand the health of the sensor.

Although limitations on the magnitude and duration of the current pulsehave mostly to do with experimental convenience, the magnitude of thecurrent pulse may, for example, be chosen to correspond to applicationof a reasonably expected amount of target gas.

Sensor presence and health may be determined by the shape of thesensor's RC charging curve, being measured by observing the differencein sensor output at the beginning and the end of the current pulse. Ifthe sensor is absent, the observed potential is equal to that whichwould be expected based on the magnitudes of the current pulse and thesensor load resistor.

FIG. 11 illustrates a block diagram of one embodiment of an electronicinterrogation circuit as described in U.S. Pat. No. 7,413,645 and asused in several embodiments of the systems described herein. In FIG. 11,the voltage follower and the current follower sections function as knownto one skilled in the art. See, for example, A. J. Bard and L. R.Faulkner, Electrochemical Methods: Fundamentals and Applications, JohnWiley & Sons: New York (1980), the disclosure of which is incorporatedherein by reference. The voltage follower maintains a constant potentialbetween the reference electrode (R) and the working electrode (W). Thecurrent follower buffers and amplifies currents which flow in theelectrochemical sensor between the counter electrode (C) and the workingelectrode (W). In an number of embodiments, the current pump applieselectronic interrogation to the sensor by forcing a known current toflow between the counter electrode (C) and the working electrode (W).

Following an electronic interrogation test as described above, the usermay, for example, be prompted to perform an exhaled breath test or a“bump check” hereof (without calibration gas) by exhaling closely intothe instrument face. Embedded instrument software observes the resultingsignal on, for example, second working electrode 250 b (designed torespond to some driving force/variable change associated with exhaledbreath such as a change in oxygen concentration). In the embodiment ofsensor 210, the observed response is decreased oxygen content in exhaledhuman breath. The embedded instrument control software compares theresult of the electronic interrogation test and the result of theexhaled breath test to established parameters. If the responses ofeither the electronic interrogation test or the exhaled breath test failto meet these pre-established criteria, the instrument may prompt theuser to perform a full calibration or some other maintenance. If theresults of both the electronic interrogation test and the exhaled breathtest meet or exceed the pre-established criteria, the instrument mayindicate to the user that it is functioning properly and is ready fordaily use.

The foregoing description and accompanying drawings set forth thepreferred embodiments of the invention at the present time. Variousmodifications, additions and alternative designs will, of course, becomeapparent to those skilled in the art in light of the foregoing teachingswithout departing from the scope of the invention. The scope of theinvention is indicated by the following claims rather than by theforegoing description. All changes and variations that fall within themeaning and range of equivalency of the claims are to be embraced withintheir scope.

What is claimed is:
 1. A method of testing a system having at least oneelectrochemical sensor for detecting an analyte gas within a housing ofthe system, the housing of the system having an inlet, comprising:exhaling in the vicinity of the inlet of the housing of the system; andmeasuring a response to exhaled breath to test one or more transportpaths of the system.
 2. The method of claim 1 wherein measuring theresponse to exhaled breath comprises measuring the response of a sensorwithin the housing of the system that is responsive to the presence ofexhaled breath.
 3. The method of claim 2 wherein the sensor responsiveto the presence of exhaled breath comprises an electrochemically activeelectrode responsive to a gas within exhaled breath.
 4. The method ofclaim 3 wherein the electrochemically active electrode is responsive tocarbon dioxide.
 5. The method of claim 3 wherein the electrochemicallyactive electrode is responsive to oxygen.
 6. The method of claim 3further comprising: simulating the presence of the analyte gaselectronically; and measuring a response of the electrochemical sensorto the electronic simulation.
 7. The method of claim 6 wherein aconstant current is caused to flow between a first working electrode anda counter electrode of the electrochemical sensor and the measuredresponse is a potential difference.
 8. The method of claim 6 wherein aconstant potential difference is maintained between a first workingelectrode and a counter electrode of the electrochemical sensor and themeasured response is a current.
 9. The method of claim 6 wherein theelectrochemical sensor is an amperometric sensor.
 10. The method ofclaim 6 wherein the electrochemical sensor comprises a first workingelectrode responsive to the analyte gas and a second working electroderesponsive to a gas within exhaled breath.
 11. The method of claim 10wherein the electrochemical sensor comprises a sensor housing comprisingat least one inlet into an interior of the sensor housing, the firstworking electrode and the second working electrode being positionedwithin the sensor housing.
 12. The method of claim 11 wherein each ofthe first working electrode and the second working electrodeindependently comprises an electrocatalytically active materialdeposited upon a porous membrane through which gas can diffuse.
 13. Themethod of claim 3 wherein the electrochemically active electrodecomprises a wire or a ribbon.
 14. The method of claim 13 wherein thewire or the ribbon comprises an electrocatalytically active materialdeposited thereon.
 15. The method of claim 13 wherein theelectrochemically active electrode comprises a section of increasedsurface area per unit length in operative connection with the wire orthe ribbon.
 16. The method of claim 15 wherein the section of increasedsurface area comprises an electrocatalytically active material depositedthereon.
 17. The method of claim 11 wherein the first working electrodeand the second working electrode form an interdigitated electrodesystem.
 18. The method of claim 11 wherein the first working electrodeis supported upon a first side of an annular support through which gasis transportable and the second working electrode is supported upon asecond side of the annular support.
 19. The method of claim 18 whereinthe annular support is gas permeable or gas porous.
 20. The method ofclaim 19 wherein the first electrode is formed to have a generallyannular cross-sectional shape and the second electrode is formed to havea generally circular cross-sectional shape.
 21. The method of claim 2wherein the sensor responsive to the presence of exhaled breath is thesame as the at least one electrochemical sensor, which comprises a firstelectrochemically active electrode responsive to a gas within exhaledbreath and responsive to the analyte gas.
 22. The method of claim 21wherein the system further comprises a second electrochemically activeelectrode responsive to a gas within exhaled breath and responsive tothe analyte gas.
 23. The method of claim 22 further comprisingalternating the function of the first electrically active electrode andthe second electrically active electrode between sensing the gas withinexhaled breath and sensing the analyte gas.
 24. A system, comprising: asystem housing, at least one inlet formed in the system housing, atleast one electrochemical sensor for detecting an analyte gas within thesystem housing and at least one sensor responsive to the presence ofexhaled breath within the system housing.
 25. A system for detecting atleast one analyte gas, comprising: a system housing comprising an inletsystem; an electrochemical gas sensor within the housing and in fluidconnection with the inlet system, the electrochemical sensor beingresponsive to the at least one analyte gas, and at least one othersensor within the housing and in fluid connection with the inlet system,the at least one other sensor being responsive to at least one drivingforce created in the vicinity of the inlet system other than byapplication of the at least one analyte gas or a simulant gas to whichthe electrochemical sensor is responsive to provide an indication of astate of a transport path between the inlet system and theelectrochemical gas sensor.